Motor driven fluid compressors, or other pump-type machinery, generally have their loading controlled either by control of the drive motor, or by controlling the fluid circuit by throttling the inlet to the compressor or by on-off bypassing of the outlet of the compressor to some earlier compressor point or stage approaching the compressor inlet. In some systems the control is automatically centered about a setpoint, and may be proportional to the displacement of the momentary operating point from the setpoint.
U.S. Pat. Nos. 3,332,605 to Huesgen and 3,743,442 to Wilson show systems in which discharge pressure is used to load and unload a compressor by throttling the inlet thereto or by bypassing the outlet to the inlet. U.S. Pat. No. 3,961,862 to Edstrom et al shows a mechanical system for throttling the inlet to a screw compressor according to measured outlet pressure. U.S. Pat. No. 2,613,026 to Banks shows a mechanical system for throttling the inlet flow according to the sensed inlet pressure, and U.S. Pat. No. 3,535,053 to Jednacz shows a system for throttling the inlet flow in response to the discharge temperature of the fluid compressed.
U.S. Pat. No. 3,535,053 to Jednacz further shows the concept of measuring current to a compressor drive motor and using this measurement to limit loading of the compressor when the current to the drive motor reaches a predetermined level. U.S. Pat. No. 3,743,442 to Wilson shows a similar system for providing drive motor overload protection, while U.S. Pat. No. 3,088,655 to Miller provides for compressor shut-down when the drive engine begins to overheat.
U.S. Pat. No. 3,478,731 to Morton et al teaches a system for starting up an engine driven compressor using a fixed sequence and having means for tripping the start cycle in the event of a malfunction. The system also uses a pre-lube step, and in this regard, U.S. Pat. No. 3,957,395 to Ensign is also of interest and provides various malfunction sensitive circuitry. U.S. Pat. No. 3,291,146 to Walker provides sequentially programmed start-up and shut-down cycles, for a steam turbine instead of a compressor however.
U.S. Pat. No. 3,673,811 to Adams provides an oil pressure failure sensor with a feature to allow start-up at a time when there has been no oil pressure for a while. U.S. Pat. Nos. 3,088,655 to Miller, 3,232,519 to Long and 3,987,620 to Giordano et al provide warnings of malfunctions based on temperatures, pressures, overload, etc., and provide indicators for showing the nature of the problem. U.S. Pat. No. 3,934,238 to Pavlou includes apparatus for measuring pressure drop across an oil filter as an indication of the degree of clogging of the filter.
The present invention provides improvements over the showings of the aforementioned prior art, which improvements are summarized in the objects of the invention set forth hereinbelow, and as illustrated in an embodiment using a screw compressor of the general type shown in U.S. Pat. No. 3,986,801 to Garland.